Temperature control system for a permanent magnetic system

ABSTRACT

A temperature control system for a permanent magnetic assembly in a permanent magnetic system comprises thermoelectric heat pumping devices which are in thermal contact with a yoke of the permanent magnet system. The heat pumping devices are controlled by an electronic circuit which controls the current that flows through them, so that the desired temperature of the permanent magnetic assemblies is achieved. Each thermoelectric heat pumping device includes a spring-loaded plunger which is mounted in a hole in the yoke. 
     Heat is exchanged from or to the yoke via ambient air by natural convection or by forced air. Alternatively heat may be exchanged from the yoke via a cooling water circuit.

BACKGROUND OF THE INVENTION

The present invention relates to a temperature control system for apermanent magnet system and in particular it relates to systems used formagnetic resonances imaging (MRI).

In such systems a magnetic field with a high degree of stability andhomogeneity is essential for the successful application of a number ofanalytical techniques such as MRI. MRI systems comprising large amountsof permanent magnetic material can be used for the generation of such amagnetic field.

Most permanent magnetic materials with sufficient energy density to beuseful for this application are generally very sensitive to temperaturevariations. The effect of the temperature variations on the airtemperature of the permanent magnetic assembly can be reduced totolerable levels by controlling the temperature of the permanentmagnetic assembly. Thus, the temperature control of existing permanentmagnet MRI assemblies rely on raising the temperature of the permanentmagnetic assemblies so that heat flows to the ambient air. Thistechnique, however, raises the temperature of the permanent magneticassemblies and results in a significant reduction of field due to thelarge temperature coefficient of the permanent magnetic materials.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a system forcontrolling accurately the temperature of the permanent magneticassemblies.

According to the present invention there is provided a temperaturecontrol system for a permanent magnet system having at least onepermanent magnetic assembly, in which the permanent magnet assembly ismounted in thermal contact with a yoke via a plurality of thermoelectricheat pumping devices. At least one temperature sensor is connected to anelectronic control circuit; and the thermoelectric heat pumping devicesare connected to and controlled by the electronic control circuit whichcontrols the current passing through the thermoelectric heat pumpingdevices in dependence upon the temperature. In this way, the permanentmagnet assembly is maintained at a desired temperature enabling thepermanent magnet system to generate an accurate magnetic field.

According to an aspect of the present invention a thermoelectric heatpumping device is provided with a spring loaded plunger to providemechanical pressure on the thermoelectric heat pumping device so that agood thermal contact between the thermoelectric heat pumping device andthe permanent magnet assembly is achieved.

According to a further aspect of the present invention the thermalcontact between the plunger and the yoke is achieved by a sliding fit ofone end of the plunger with a hole in the yoke into which the said oneend of the plunger is fitted.

According to still yet a further aspect of the present invention thermalgrease is enclosed within said hole, and the plunger is provided with aventilation hole to prevent displacement of the grease by trapped air assaid one end of the plunger is inserted into said hole.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a permanent magnetic assembly,

FIG. 2 shows an example of a plunger, and,

FIG. 3 shows a view of a C-shaped magnetic assembly having a heatexchanger connected thereto.

DETAILED DESCRIPTION OF THE INVENTION

Permanent magnetic assemblies are in general mounted on a largestructure which, apart from providing mechanical support, also providesa magnetic return path for the magnetic field. This structure iscommonly referred to as a yoke. The yoke is, in general, a structure ofconsiderable size and has a significant thermal capacity.

In accordance with the present invention, the temperature of thepermanent magnetic assembly is controlled by transferring heat to andfrom the yoke via the thermoelectric heat pumping devices.

Referring to FIG. 1, a permanent magnetic assembly 1 consists of layerof permanent magnetic material 2 glued between plates 3,4. At leastplate 3 has good thermal conductivity. The permanent magnetic assemblyis bolted to the yoke 5 with bolts 6 having low thermal conductivity.Spacers 7, made of poor thermal conducting material, maintain a space 8between the permanent magnetic assembly and the yoke 5. This space actsas a thermal barrier, and may be filled with a thermal insulatingmaterial. One or more thermoelectric heat pumping devices 9 are mountedinside the space 8. The mechanical pressure required to ensure goodthermal contact with the permanent magnetic assembly is provided byspring loaded devices 10, referred to as plungers. A plunger is shown ingreater detail with reference to FIG. 2.

In FIG. 2, identical parts shown in FIG. 1 bear the same referencenumeral.

The top end of the plunger provides a sliding fit with a hole 11 in theyoke 5. Thermal conducting grease 20 is inserted into a gap between thehole and the plunger to ensure good thermal contact between the plungerand the yoke. The top end of the plunger is provided with a ventilationchannel 12 to prevent the displacement of grease by trapped air wheninserting the plunger into the counter bored hole 11 inside the yoke 5.Making the thermal contact at the top end of the plunger avoids largetemperature gradients near the thermoelectric heat pumping devices,which would reduce the performance of the heat pumping devices. Theradial gap between the plunger 10 and the yoke 5, apart from the slidingfit at the top, acts as a thermal barrier. The gap is used toaccommodate a spring 13 that provides mechanical pressure on the plunger10. The permanent magnetic assemblies are fitted with temperaturesensors 14 which are connected to an electronic control circuit 15. Thelatter in turn is connected to the thermoelectric heat pumping devicesvia a connection 15 a, and controls the current to the thermoelectricheat pumping devices. Depending on the polarity of the current in thethermoelectric heat pumping devices, the latter can be made to pump heatto or from the permanent magnetic assemblies, which will increase ordecrease the temperature respectively. The heat transferred to the yoke5 including the heat due to dissipation of the thermoelectric heatpumping devices must be exchanged with the environment of the magnet.This can be achieved by natural conduction from the yoke 5 to surfaceair. Alternatively heat could be exchanged to a liquid cooling circuitas shown in FIG. 3.

Referring to FIG. 3, a heat exchanger is provided which is mounted tothe yoke 5. The thermal contact between the exchanger and the yoke couldhave a moderate thermal resistance. This, together with the thermalcapacitance of the yoke constitutes a certain amount of filtering whichmakes the system less sensitive to variations in the temperature of thecooling water. The thermal resistance could be achieved by a pad ofthermally insulating materials 17 inserted between the first heatexchanger at 16 and the yoke 5. A second heat exchanger 18 with goodthermal contact can be also mounted to the yoke 5, and the flow ofcooling liquid to the second heat exchanger 18 is controlled by a valve19 in one of the conduits feeding the heat exchangers. The second heatexchanger 18 can be used to settle a temperature of the yoke to thecooling water temperature in a reduced amount of time. This results in atime saving during the installation process of the magnet on site.

Each permanent magnetic assembly may be controlled by the same, common,electronic control circuit, or may each be controlled independently byseparate electronic control circuits.

While the present invention has been described with reference to a Cshaped magnet it will be appreciated that it is applicable to otherforms of magnet.

The present invention allows regulation of the temperatures of thepermanent magnetic material above, below or at the temperature of theheat source/sink of the magnetic system. When permanent magneticmaterials with a significant negative temperature coefficient are used,such as NdFeB or ferrite, the problem of a reduced performance of thepermanent magnetic material due to raised temperature operation isavoided resulting in a saving of permanent magnetic material.Furthermore, the devices which apply pressure to the thermoelectric heatpumping devices do not impair the magnetic efficiency of a ferromagneticyoke if the pressure devices are made of ferromagnetic material.

The cooling circuit may have a control valve in each leg, and the boreof the conduit to the second heat exchanger may be wider than that ofthe conduit to the first heat exchanger.

The system described above, operates at a near ambient temperature andtherefore avoids the need to raise the system above ambient temperatureas in the prior art which leads to a consequential loss of performance.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A temperature control system for a permanentmagnet system having at least one magnetic assembly, which is mounted inthermal contact with a yoke via a plurality of thermoelectric heatpumping devices, and having at least one temperature sensor connected toan electronic control circuit, said thermoelectric heat pumping devicesbeing connected to and controlled by said electronic control circuitwhich controls the current passing through said thermoelectric heatpumping devices dependent upon sensed temperature, so that the permanentmagnet assembly is maintained at a desired temperature enabling thepermanent magnet system to generate a stable and homogeneous magneticfield.
 2. A temperature control system as claimed in claim 1, whereineach thermoelectric heat pumping device is provided with a spring loadedplunger to provide mechanical pressure on the thermoelectric heatpumping device so that a good thermal contact between thetheremoelectric heat pumping device and the permanent magnet assembly isachieved.
 3. A temperature control system as claimed in claim 2, whereinthe thermal contact between the plunger and the yoke is achieved by asliding fit of one end of the plunger with a hole in the yoke into whichthe said one end of the plunger is fitted.
 4. A temperature controlsystem as claimed in claim 3, wherein thermal grease is enclosed withinsaid hole, at the top of the plunger, and the plunger is provided with aventilation channel to prevent displacement of the grease by trapped airas said one end of the plunger is inserted into said hole.
 5. Atemperature control system as claimed in claim 1, wherein heat isexchanged from or to the yoke by ambient air by natural convection or byforced air.
 6. A temperature control system as claimed in claim 1,wherein heat is exchanged for or to the yoke via a cooling watercircuit.
 7. A temperature control system as claimed in claim 6, whereinthe cooling water circuit comprises a first heat exchanger arranged inparallel with a second heat exchanger, and flow of cooling water iscontrolled to said second heat exchanger by at least one valve, saidsecond heat exchanger being in good thermal contact with the yoke and sused to minimise the temperature difference between the yoke and thecooling circuit in a reduced time when the permanent magnet system isinstalled.
 8. A temperature control system as claimed in claim 7,wherein a layer of material having low thermal conductivity is appliedbetween the yoke and the first heat exchanger to reduce variations inthe heat flow and temperature in the yoke due to variations in thetemperature of the cooling circuit.
 9. A temperature control system asclaimed in claim 2, wherein each permanent magnetic assembly iscontrolled by common electronic control circuit or is independentlycontrolled by a separate electronic control circuit.
 10. A temperaturecontrolled permanent magnet system, comprising: at least one permanentmagnet assembly; a yoke; a plurality of thermoelectric heat pumpingdevices mounted in heat flow communication between said at least onemagnet assembly and said yoke, said heating pumping devices beingoperative to pump heat between said at least one magnetic assembly andsaid yoke, in a direction dependent on a direction of current flowtherein; at least one temperature sensor mounted to said at least onemagnet assembly, for sensing a temperature thereof; and means formaintaining said at least one magnet assembly at a desired temperature,by controlling a flow of electric current through said plurality ofthermoelectric heat pumping devices as a function of temperature of saidat least one magnet assembly sensed by said at least one temperaturesensor, whereby said permanent magnet system is enabled to generate astable and homogeneous magnet field.
 11. The system according to claim10, wherein said means for maintaining comprises an electronic controlcircuit which is coupled to receive output signals from said at leastone temperature sensor and is connected to said plurality ofthermoelectric heat pumping devices.
 12. The system according to claim11, wherein each of said thermoelectric heat pumping devices includes aspring loaded plunger which is in thermal contact with said at least onemagnet assembly at a first end of said plunger, and is in thermalcontact with said yoke at a second end of said plunger.
 13. The systemaccording to claim 12, wherein said spring loaded plunger is springbiased against said permanent magnet at said first end, and is insertedin a hole in said yoke at said second end.
 14. The system according toclaim 13, wherein thermal grease is enclosed in said hole, adjacent saidsecond end of said plunger.